The invention relates to fluorocopolymers, as defined herein to denote any fluoropolymer containing tetrafluoroethylene monomer units and at least or more than 1.0% by weight of units of at least one other comonomer,* polymerized to produce an expandable tetrafluoroethylene copolymer of the fine powder type. A process of polymerization of these monomers is described, as well as the porous products produced by expansion (stretching under controlled conditions) of the aforesaid copolymers. * See, e.g., Fluoroplastics—Vol 1: Non-Melt Processible Fluoroplastics; Williams Andrew, Inc., Norwich, N.Y., at p. 19 (2000); see, also, ISO 12086.
Techniques for the dispersion polymerization of tetrafluoroethylene (TFE) monomer are known. Dispersion polymerization of TFE produces a resin that has come to be known as “fine powder”. See, e.g., U.S. Pat. No. 4,016,345 (Holmes, 1977). In such processes, generally, sufficient dispersing agent is introduced into a water carrier such that, upon addition of tetrafluoroethylene monomer in the presence of a suitable polymerization initiator and, upon agitation and under autogenous tetrafluoroethylene pressure of 10 to 40 kg/cm2, the polymerization proceeds until the level of colloidally dispersed polymer particles is reached and the reaction is then stopped.
In contrast, particulate tetrafluoroethylene resins have also been produced by a process of suspension polymerization wherein tetrafluoroethylene monomer is polymerized in a highly agitated aqueous suspension in which little or no dispersing agent is employed. The type of particles produced in suspension polymerization has been termed “granular” resin or “granular powder”. See, e.g., U.S. Pat. No. 3,655,611 (Mueller, 1972).
For both polymerization processes, copolymerization of tetrafluoroethylene with various fluorinated alkyl ethylene comonomers has been described. See, for example, U.S. Pat. No. 4,792,594 (Gangal, et al., 1988). However, the present invention relates, specifically, to the aqueous dispersion polymerization technique, in which the product of the polymerization reaction is the copolymer of the invention dispersed within an aqueous colloidal dispersion. In this process, tetrafluoroethylene monomer is pressured into an autoclave containing water and polymerization initiators, along with paraffin wax to suppress coagulum formation and an emulsifying agent. The reaction mixture is agitated and the polymerization is carried out at suitable temperatures and pressures. Polymerization results in the formation of an aqueous dispersion of polymer particles, and the dispersed polymer particles may subsequently be coagulated by techniques known in the art to obtain what has become known as the fine powder form of the polymer.
Various prior patents have disclosed techniques for the homopolymerization of tetrafluoroethylene and for the polymerization of TFE with small amounts (<1.0% by weight) of other monomers. Among those are included U.S. Pat. No. 4,576,869 (Malhotra, 1986) and U.S. Pat. No. 6,177,533B1 (Jones, 2001).
Fine powder resins are known to be useful in paste extrusion processes and in stretching (expansion) processes in which the paste-extruded extrudate, after removal of extrusion aid lubricant, is stretched to produce porous, strong products of various cross-sectional shapes such as rods, filaments, sheets, tubes, etc. Such a stretching process is disclosed in the pioneering U.S. Pat. No. 3,953,566 (Gore, 1976), assigned commonly with the instant invention.
The expansion process as it applies to fluorocarbon polymers is fully described in the aforesaid '566 patent, and that process has come to identify what is currently termed the “expanded” form of TFE fluoropolymers, and will serve to define what is meant herein as an expanded or expandable TFE polymer or copolymer.
The term “copolymer” as it has been used in connection with fluoropolymers in the prior art has been inconsistently applied. For all purposes herein, as set out in the Fluoroplastics text cited above and in the ISO 12086 classification cited above, the normal convention of polymer science will be followed, and the term “copolymer” will apply to any fluoropolymer containing more than 1.0% by weight of at least one comonomer in addition to TFE. A fluoropolymer containing less than 1.0% comonomer is properly categorized as a “modified” homopolymer (Id.), although the term “copolymer” has been misapplied in the literature when referring, in fact, to “modified” homopolymers. One must examine each particular instance of such use to determine the actual concentrations of comonomers employed to determine whether, in fact, the referenced composition is a “modified” homopolymer or a true copolymer, that is, whether or not the polymeric product, in fact, contains more than 1.0 weight percent comonomeric units.
By definition herein, the invention provides a true TFE copolymer, of the fine powder type, that is expandable, as defined above, to produce useful, expanded TFE copolymeric products.
U.S. Pat. No. 4,837,267 (Malhotra, 1989) discloses a three-component composition termed “core-shell TFE copolymers”, which are described as non-melt processible, including chlorotrifluoroethylene (CTFE) monomer residing in the core and having recurring units of a comonomer of perfluoro(n-alkylvinyl)ether of 3-7 carbon atoms (col. 1, lines 45-55). The total comonomer content in the particles is said to be between 0.001 and 2 weight percent. The examples presented all relate to terpolymers having comonomeric concentrations much less than the range described, namely 0.23% CTFE and 0.0145% PPVE (total of 0.2445 wt %) in Example 1, and 0.13% HFP and a minute, undeterminable amount of PPVE in Example 2. The stated upper limit of 2% is therefore unsupported by the specification and examples presented. Moreover, there is no disclosure or suggestion in the '267 patent of an expanded or an expandable TFE copolymeric composition.
Japanese Patent Application (Kokai) 2005-306033A, published Nov. 4, 2005, discloses thin films of PTFE which are said to be non-porous, non-gas-permeable (p. 5), and to contain “trace monomer units” in the range of 0.001-2 mol % (p. 7) described as “modified” PTFE. The objective of the invention is said to be obtained by “heat treatment” of “porous PTFE resin film” to render the film “substantially nonporous”. There is no disclosure or suggestion in this reference of a porous, expandable TFE copolymeric composition.
U.S. Pat. No. 4,391,940 (Hoechst, 1983) discloses and describes a partially modified tetrafluoroethylene polymer having a “three-shell” particle structure. The resins are said to be suitable for paste extrusion to produce cable insulation and highly stretchable, unsintered tapes ('940 patent, Abstract). This patent describes fluorinated modifying monomers which are capable of copolymerizing with tetrafluoroethylene, such as perfluoropropene, perfluoroalkyl vinyl ether, and halogen-substituted or hydrogen-substituted fluoroolefins. The specification cautions that the total amount of the comonomer modifying agent should be so low that the specific properties of the pure polytetrafluoroethylene are retained, that is, there remains no possibility of processing from the melt because of the extremely high melt viscosity for such modified polymers. ('940 patent, col. 1, l. 62 et seq.) Products disclosed include modified polymer particles having a core of a polymer of “0.05 to 6% by weight” of at least one modifying fluoroolefin comonomer, a first, inner shell, immediately adjacent the core, of TFE units, and a second, outer shell, immediately adjacent the inner shell, of a polymer comprising “0.1 to 15% by weight” of units of at least one modifying fluoroolefin (col. 3, l. 5, et seq.). Examples of the “three-shell” products provided in this reference for illustration of the principles disclosed therein show that tapes, upon stretching, after removal of lubricant, developed defects or tore completely at relatively modest stretch ratios. For example, the detailed procedure described in Example 31, at col. 14, l. 60 to col. 16, l. 6, produced a product which developed defects at a 4:1 stretch ratio and tore completely at a stretch ratio of 8:1 ('940 patent, Table III).
For comparison and to place various of the prior art disclosures in context, recently issued U.S. Pat. No. 6,841,594 (Jones, 2005) instructs that polytetrafluoroethylene (PTFE) refers to the polymerized tetrafluoroethylene by itself without any significant comonomer present, and that “modified” PTFE refers to TFE polymers having such small concentrations of comonomer that the melting point of the resultant polymer is not substantially reduced below that of PTFE. The concentration of such comonomer, consistent with prior citations above, is preferably less than 1 weight %, more preferably less than 0.5 weight %. The modifying comonomers cited include, for example, hexafluoropropylene (HFP), perfluoro(methyl vinyl ether) (PMVE), perfluoro (propyl vinyl ether) (PPVE), perfluoro (ethyl vinyl ether) (PEVE), chlorotrifluoroethylene (CTFE), perfluoro-butyl ethylene (PFBE), or other monomer that introduces side groups into the molecule. These instructions are consistent with the disclosures above and with the definitions contained herein, i.e., that the term “copolymer”, as contrasted with the term “modified homopolymer”, shall mean any fluoropolymer containing more than 1.0% by weight of at least one comonomer in addition to TFE.
U.S. Pat. No. 6,127,486 (Burger, et. al., 2000) discloses a blend of a fluoropolymer and a “thermoplastic”, wherein the “thermoplastic” is said to include a “PTFE copolymer” (col. 4, l. 46). The specification instructs that, for the resins described therein, the amount of comonomer is limited such that the [modified] PTFE exhibits properties of “not being processable in the melt.” (Emphasis in original). The PTFE is referred to as modified PTFE “in which the comonomers are contained in an amount below 2, preferably 1 wt. % in PTFE.” (Col. 4, l. 50) No examples are provided of any copolymer having greater than 1.0 weight % of an additional comonomer, and the patent concerns blends of polymers, a different physical form entirely from the true copolymers which form the subject matter of the present invention.
Another recent reference, Japanese Patent Application No. 10-243976 (Asahi Glass Co., Ltd., claiming priority to Dec. 26, 1997) is still further instructive of the state of the art in the field of copolymers and modified homopolymers of TFE. That patent application, titled “Tetrafluoroethylene Copolymer and Application Thereof”, contains claims to polymers having, inter alia, additional comonomer content in the range of 0.005 to 0.05 mol % (about 0.012 to 0.123 wt %). The patent discusses known copolymerization techniques and discloses that a further, related Japanese application, JP (Kokoku) 3-66926, proposes a method for modifying PTFE by employing Rf—CH═CH2 (where Rf is a C1-10 perfluoroalkyl group) as a comonomer. In the proposed method, the comonomer is continuously added during the polymerization process in order to enhance modification in the initial period. The modification is said to be primarily performed in order to improve the paste extrudability of fine powders, for example, to reduce extrusion pressure, and the content of polymerization units based on comonomers, while less than 0.5 wt %, is “still comparatively high in substantial terms” (0.1 wt % or higher). Consequently, the product has substantially no melt moldability and possesses markedly reduced crystallinity. The reference describes “another draw-back”, that such modified PTFE becomes less heat-resistant because of the structure of the comonomers introduced. Finally, the Asahi patent application concludes, quoting therefrom:                In addition, the comonomer structure impairs molecular orientation, causing breakage during stretching and making the product substantially unusable for the manufacture of stretched porous articles.        An object of the present invention is to provide a PTFE product that has excellent extrudability, can be uniformly stretched, and yields high-strength porous articles.        
This objective is then said to be obtained by limiting the introduction of polymerization units based on comonomers copolymerizable with TFE to an amount that has no discernible effect on processibility.
Specifically, the Asahi application provides a product of TFE and a fluorinated comonomer expressed by the general formula CH2═CH—Rf (where Rf is a C1-10 perfluoroalkyl group, wherein this polymer contains 0.005 to 0.05 mol % polymerization units based on the fluorinated comonomer. Further, a porous polymer article is provided, obtained by a process in which a powder composed of the aforementioned modified PTFE is paste-extruded and then stretched at a temperature of 250° C. or higher. This reference, however, specifically cautions against polymerization in which the amount of copolymerized monomer exceeds certain limits. The application states, again quoting directly:                The content of the polymerization units based on fluorinated comonomer in the present invention must be rigorously controlled because of considerations related to stretchability. The content of the units in the PTFE must fall within a range of 0.005 to 0.05 mol %. A content above 0.05 mol % brings about a slight reduction in polymer crystallinity, results in a lower paste extrusion pressure, and has a markedly adverse effect on stretchability. A content below 0.005 mol % makes it substantially more difficult to improve the physical properties of a stretched article or to obtain other modification effects. A range of 0.01 to 0.04 mol % is particularly preferred.        
This, again, is consistent with the other teachings of the prior art references discussed hereinabove. In Example 4 of this Asahi reference, in which a “high” content (by applicant's definition), 0.42 wt %, of perfluorobutylethylene comonomer was employed, the paste extrusion pressure was desirably low, and “excellent” extrudability was obtained. However, a test specimen, on stretching, broke. The specification discloses, at this “high” level of comonomer concentration of 0.42 wt %, “ . . . breakage occurred during stretching, and it was impossible to obtain a porous article.” (p. 12, §0050).
In spite of these cautionary teachings, and in contrast thereto, the present invention is directed to true TFE copolymers, all containing in excess of 1.0 weight percent comonomer units, all of which are expandable to form porous expanded articles, to a process for their manufacture, and to the expanded articles produced thereby. No known prior art reference discloses or suggests such porous, expanded copolymeric articles or the resins from which they are produced.
It is wholly unexpected, and contrary to prior art teachings, that a TFE copolymer, having comonomeric unit concentrations in the high ranges claimed herein, can be expanded as disclosed hereinbelow, to and beyond a 25:1 stretch ratio, to form a uniform, viable shaped article. This synergistic result is truly surprising to one skilled in this art.